Pressurised water reactors include a primary circuit through which circulates the water cooling the core of the nuclear reactor located within the reactor vessel so as to transfer the heat drawn from the core of the nuclear reactor to the feed water which is heated and vapourised within the steam generators. The primary circuit of a pressurised water reactor includes at least one loop and generally several loops (three or four) in each of which is provided a steam generator and a primary pump which circulates the primary coolant between the reactor vessel and the steam generator. Each of the loops in the nuclear reactor's primary circuit comprises three main large diameter pipes, each of which connect to components of the primary circuit. A first large diameter pipe, or hot leg, is connected to the vessel on the one hand and a primary part of the steam generator on the other at an inlet compartment to the water chamber of the steam generator, and transfers water heated in contact with the core in the reactor vessel to the steam generator. A second pipe, called the cross-over leg, links an outlet compartment from the water chamber of the steam generator to an inlet pipe for the volute of the primary pump. A third large diameter pipe or cold leg provides a link between the outlet pipe from the volute of the primary pump and the reactor vessel. Cooling water cooled in the steam generator and drawn in by the primary pump is delivered to the reactor vessel to cool the core through the cross-over leg, the primary pump and the cold leg.
Auxiliary and back-up circuits are associated with the primary circuit to perform particular functions, either during normal operation of the nuclear reactor, or during shutdowns, or again in the course of incidents or accidents.
In particular a chemical and volume control circuit (RCV circuit) is connected to the pipes of the primary circuit to control the quantity of water present in the primary circuit, in particular by making periodical make-up injections, and the chemical composition of the reactor cooling water, which contains various additives, by sampling the primary water and re-injecting, into the primary circuit, water containing additives in the requisite quantity to ensure the required composition of the cooling water.
Among the back-up circuits associated with the primary circuit, the safety injection circuit (RIS) injects a large flow of pressurised water into the primary circuit if there is a major water loss which cannot be compensated for by the RCV circuit.
The auxiliary and back-up circuits associated with the primary circuit comprise pipes having a very much smaller diameter than the diameter of the primary pipes, and these are connected to the pipes of the primary circuit through branch connections.
In particular, the RCV circuit, which consists of three inch (76 mm) pipe, is connected to the cold leg by a branch connection in a part of the cold leg located close to the outlet pipe from the volute of the primary pump. Water in the RCV circuit is reinjected into the primary circuit through the cold leg branch connection with the result that the inner surface of the cold leg in this reinjection area is subjected to flows of water at possibly very different temperatures depending upon the operating status of the reactor and the RCV circuit.
Some defects have been observed on the inner surface of the cold leg in these cold leg reinjection areas due to the circulation of flows of water at different temperatures, these defects being known by the name of “crazing”.
Although these defects only affect a superficial part of the inner surface of the cold legs, for safety reasons it has been felt necessary to sample sections of the cold leg affected by crazing for expert assessment, and for isolated or possibly systematic replacement of cold leg sections in the light of the results of the expert assessments.
The sections of cold leg which are sampled have a first end adjacent to the outlet pipe from the volute of the primary pump and a second end providing a connection between two successive welded sections forming the cold leg. A section of this type bounded as described above is approximately 3500 mm long and includes various branch connections in addition to the branch connection for a feed pipe to the RCV circuit, in particular a branch connection to the auxiliary spray line and the spray circuit of the pressuriser which controls pressure in the primary circuit.
Conversely the section which has to be replaced does not include the connection to the RIS circuit, the cut opposite the end of the connection to the volute of the pump being made upstream of the connecting pipe to the RIS circuit.
Procedures for replacing the components of a primary circuit of a nuclear reactor and in particular procedures for replacing steam generators in which the pipes of the primary circuit are cut and the pipes of a new replacement steam generator are connected to the cut pipes of the primary circuit by welding are known. After the pipes have been cut and the connecting surfaces between the pipes and the replacement steam generator pipes have been machined, the steam generator is placed in position so that the connecting surfaces form bevels between them, through the full thickness of the pipes and branches, into which weld metal is laid on the outside of the pipes and the bevel using a process of the orbital TIG type. Generally narrow bevel welding is performed, that is to say metal is deposited on a bevel of small width, generally less than 15 mm, in which the opposite surfaces form a very small angle, less than 5°, between them.
Where such a component replacement is made, the welds are made under conditions which are the same as those used when a steam generator is initially fitted and they do not therefore have any special features which require additional inspection or machining over and above those performed in the operation of incorporating a steam generator into a nuclear power station when under construction.
Conditions are completely different when a section of pipe, and in particular a section of a cold leg, is replaced in the primary circuit which is an operation involving restoration of part of the pipe itself.
Hitherto no procedure and means have been known to ensure optimum conditions for replacing a section of a primary pipe.